Measuring Induced AC Voltage and Current
Any site with induced AC voltage to levels that require mitigation needs a product that provides AC grounding, without compromising the cathodic protection voltage. The device must be suitable for conducting steady-state AC current while blocking DC current, when connected between pipeline and ground.
The steady-state induced voltage can be measured between the two points of interest using a multimeter set to AC volts. NACE recommends that a voltage of 15V or greater be mitigated, however even lower amounts can be a hazard or cause AC corrosion.
The steady-state AC current can be measured using a clamp-on ammeter at a temporary bond established between the connection points - the pipeline and grounding system. This value can then be compared to the DEI product ratings.
Key: The DEI product rating for steady-state AC current should exceed the values measured on site.
Procedure:
| 1. Using appropriate safety precautions, connect a temporary jumper between the pipeline and the grounding system. In the case of induced AC voltage at an insulated joint, where the other side of the joint is grounded, connect the jumper across the joint. For underground transmission cable sheaths/casings, a grounding conductor will already be in place.
2. Using a clamp-on AC ammeter, read the AC rms current flowing in the conductor. If this value fluctuates daily or seasonally, several readings should be taken over time, selecting the highest reading for the rating of a DEI product. |
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3. If several readings are not possible, ratio the measured value upward to reflect the higher seasonal changes. As one major variable is the power line load current, the ratio could be determined from a comparison of the load current at the time of measurement to the power company's maximum expected load current. This would require coordination with the power company to acquire these values. Lacking any ratio information, include at least some margin between the measurement and the device steady-state AC current rating. For underground transmission cable sheaths/casings, consider imbalance and harmonics.
4. In the final installation, the DEI device is permanently connected between the pipeline and grounding system (or across the insulated joint, if applicable) using cable sized for the AC fault current expected.
Related issues:
- Full review of AC Voltage Mitigation
- AC fault current review
Reference material:
- NACE standard RP0177-2000 "Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems"